Numerous types of devices can be used to determine the speed of a vessel as it travels through a body of water. Certain devices, based upon ultrasonic principles, determine the speed of a vessel relative to the bottom of the body of water. Other types of sensors measure the relative speed through the water using, for example, a small paddle wheel or propeller. Another type of device that can determine relative speed through water is an electromagnetic flow sensor based upon the Faraday effect. The operating principle of the present invention is that of the Faraday Effect, in which a conductor moving perpendicularly to a magnetic field has induced within it an electric field that is perpendicular to both the velocity and the magnetic field directions. See The Theory of Electromagnetic Flow Measurement, J. A. Shercliff, Cambridge University Press (1962). In the case of a speed transducer, the conductor is water, with enough dissolved ions to make it sufficiently conductive.
Water speed transducers that utilize the Faraday effect are disclosed by U.S. Pat. Nos. 3,903,741 and 3,940,983 both issued to Greene. The devices disclosed by these patents are electromechanical and rely upon the rotation of a permanent magnet within a housing to produce a magnetic field. A Faraday effect transducer that uses a fixed permanent magnet is disclosed by U.S. Pat. No. 4,653,319 --Parsonage. Finally, U.S. Pat. No. 3,885,433--Marsh discloses a Faraday effect transducer that uses pairs of arcuate segment electrodes to produce a velocity signal. However, none of the transducers known in the prior art operate effectively at low speeds, and in addition many are cumbersome and not well suited for lightweight watercraft.
Several types of watercraft pose unique problems for speed determination. The shells and sculls used in competitive rowing are highly specialized, and the athletes who propel them are often closely matched in terms of physical ability. It therefore becomes crucial to minimize both the drag and weight of the shell. However, accurate speed information can be extremely useful both during training and during a race. Similarly, small sail craft require lightweight, low drag hulls and are of highly refined design within several classes. Thus, in either instance, it would be desirable to measure speed accurately without interfering with the operation of the vessel and without incurring excessive drag or adding excessive weight. Accordingly, it is an object of the present invention to provide a lightweight, streamlined electromagnetic transducer for use in determining the speed of watercraft. The relatively low speeds at which rowing shells and small sailboats travel, combined with the size restrictions discussed above, pose a problem wherein the electromagnetic flow sensors found in the prior art produce signals of insufficient power to be discriminated from environmental noise and other effects found in these applications. It is thus another object of the present invention to provide an amplifier circuit that provides sufficient amplification while eliminating the effects of background noise and the like.
Additionally, speed transducers as well as other types of transducers are mounted to a hull below the water line so they project beyond the surface of the hull. These transducers are typically housed or covered by a structure such as a fin that exhibits a low hydrodynamic drag coefficient. However, such projecting fins are easily damaged by obstructions in the water, e.g., logs, rocks, pilings, or by contact with objects when the watercraft is out of the water. In some instances, the fin is fastened using breakaway bolts designed to permit the fin to "snap off" rather than damaging the fin or the hull. These designs, however, do not provide an adequate solution for several reasons. First, the broken off fin is easily lost or further damaged when detached from the hull. Second, the through-hull electrical connection is broken and is typically not easily repairable. This latter problem results from the electrical connection being "potted" within the fin to protect it from water damage. As well known to those of ordinary skill, a potted connection comprises a connector that is encapsulated to protect it from the environment. For example, an electrical connector might be covered with epoxy in order to create a watertight connection. Potted connections, however, are not easily repaired or disconnected. Thus, when a transducer has been broken free from the hull, it cannot be reconnected without returning the fin to a repair facility to remove and repot the connector. The potted connection also prevents transducers from being exchanged between boats. It would therefore be desirable to provide an electrical connection to a hull-mounted transducer or other device that is watertight and can be easily reconnected should the transducer be broken off or otherwise removed from the hull. Accordingly, it is an object of the present invention to provide such improved transducer connections.